The present invention relates to microwave couplers and more particularly to microwave branching couplers such as power splitters and combiners.
For proper operation, many microwave radio frequency (RF) circuits must have two isolated signals derived from a single signal or must have two such signals combined into one. These signals, may need to be equal in phase or may need to be in phase quadrature. Branching circuits provide such signal conversion. Power splitters are used to produce two such signals from one. Power combiners are used to combine two such signals into one. In prior art strip transmission line microwave circuits such branching circuits are usually either of the interdigitated or Wilkinson type. A strip transmission line circuit is one in which microwave signals are carried by narrow strip conductors spaced by a dielectric region from at least one substantially wider ground conductor. A microstrip circuit is a special case of a strip transmission line circuit in which the strip conductors are disposed on one major surface of a solid dielectric substrate and a single wider ground conductor is disposed on the other major surface of that substrate.
Both interdigitated and Wilkinson couplers require a relatively large substrate surface area for their implementation. This is a significant drawback where the microwave circuit is intended for use in a system which must be compact or lightweight or both. A further drawback of such couplers is the fact that they are lossy with the result that both of the signals emerging from an equal-power power splitter have powers which are more than 3 dB below the level of the incoming signal. Similarly, the power emerging from the common terminal of a combiner is less than the sum of the powers entering the branch terminals. Compact branching circuits are needed which produce a lesser loss or some gain.